The present invention concerns a microreactor for the desalination of saline water according to claim 1 and a desalination method according to claim 15.
No society can survive without fresh water. Among the entire amount of water available on earth only 1% is available for plants, animals and humans, 97% of water is found in the oceans and 2% remain stored in the ice at the poles and in glaciers.
The overuse of fresh water reserves is becoming an increasingly greater problem in many parts of the world. Estimates predict that in 2030 two thirds of the world's population will suffer water shortages. According to UNESCO, in 2025, 75% of the world's population will live less than 60 km from the sea, the simplest and most efficient method to obtain fresh water is therefore to desalinate sea water.
There are two major categories of desalination techniques: thermal processes or membrane-based processes.
Among the thermal techniques, the most widely used known methods are:
Single effect distillation: this technique gives a low yield.
Multiple effect distillation (MED) well known in the prior art. This method strongly increases yield compared with single effect distillation.
Another significant improvement is brought by the so-called vapour compression technique.
A process using multi-stage flash distillation (MSF): one of the major problems with MED techniques is that of scaling. To overcome this problem, the MSF technique was developed in the 1960s.
A process using mechanical vapour compression (MVC).
Among the membrane processes, the most widely used known methods are: electrodialysis (ED), reverse osmosis (RO) and membrane distillation (MD).
The above-described processes are the most important and together they represent more than 90% of the desalination methods used over the world. There are other known methods of lesser importance, these being:
Crystallization processes: freeze desalination; gaseous hydration methods
Wetting methods: wetting-dewetting; solar evaporation—greenhouse effects; evaporation.
Ion exchange method
Liquid-liquid extraction method.
Flow-Through Capacitor method (FTC): this method is fairly similar to the electrodialysis method in that the ions are separated under the effect of an electrical field. However, contrary to electrodialysis, the FTC method does not use membranes but the ions are collected on porous electrodes e.g. aerogels. The applied voltage is moderated to prevent the occurrence of electrochemical reactions. The method is composed of two cycles. At the first cycle, called desalination cycle, an electrical potential difference is applied to the electrodes between which the saline water circulates. The ions migrate towards their respective electrodes and attach themselves thereto. Desalinated water leaves via the outlet of the system. After a certain time, the electrodes are saturated with ions. At this time, the second cycle called regeneration cycle is initiated: the voltages are reversed at the electrodes and the ions are released. Strongly saline water leaves the system. The first cycle is then recommenced.
DE 20315557 describes desalination equipment in which saline water passes by electrodes that are electrically charged either with positive charges or with negative charges. The electrodes are coated with a plastic insulator and they are also coated with an ion exchange layer, either anodic or cathodic. These layers adsorb sea water Na+ and Cl− ions, which amounts to a first essential difference compared with the present invention which does not comprise any adsorption-release operation. DE 20315557 shows that the water flows continuously on the electrodes. On account of adsorption of the ions, the electrodes become charged and lose their efficacy. The polarity must then be reversed to release the ions: this implies that the flow of desalinated water is discontinuous, in sequenced batches, which amounts to a second essential difference compared with the present invention which uses a continuous flow of desalinated water. DE 20315557 does use electrodes that are insulated from the water by plastic. Nonetheless, the form of the electrodes fully differs from those of the present invention. In DE 20315557 the form used is a metal bar/wire form coated with plastic. In addition, on top of the plastic there is an ion adsorption layer. The plastic layer to a large extent, therefore acts as substrate for this absorption layer. The electrodes are then assembled and positioned in a channel in which the water to be desalinated circulates. In the present invention, the electrodes are of rectangular cross-section and are mounted in the walls of the channel. The plastic is used solely as insulator and does not receive an ion absorption layer. The essential difference is that in the present invention a main function of the plastic is to form the main channel. Also, the operating principle in DE 20315557 and of the present invention is not the same. DE 20315557, like the present invention, is based on the principle of capacitive de-ionisation. However, DE 20315557 operates along the conventional principle of this method, namely with successive adsorptions and desorptions and the flow of desalinated water is not continuous, contrary to operation in the present invention.
US 2014/197034 describes a desalination method wherein the sea water circulates longitudinally in a channel having a passage with a V-shaped or Y-shaped cross-section. Conveyor belts circulate on the angled walls of the chamber. They are driven by a motor and pulley system outside the chamber. The conveyor belts enter via the top of the chamber and leave at the bottom at the point where the angled walls meet up. Electrodes are attached to the angled walls that are positively or negatively charged. The conveyor belts are made of plastic film. Flexible electrodes are attached thereto. When the conveyor belts are moved, the dielectric film is directed towards the wall and the flexible electrodes are directed towards the water. When passing in front of the fixed electrodes, the flexible electrodes become charged and attract the mobile ions of the saline water, this amounting to a first essential difference with the present invention which does not comprise any adsorption-release operation. These ions remain attached onto the conveyor belts until the belts leave the chamber via the bottom of a V-shaped space. The flexible electrodes are then de-charged and the ions are released and recombine to salt. The corresponding system and infrastructure are fairly complex, however it clearly arises that the desalinated water must travel a significant distance within this large-size non-watertight structure, which amounts to another essential difference with the present invention which on the contrary requires the use of microreactors of very small size, the total required capacity being achieved through multiplication of the number of micro reactors in a suitable arrangement (numbering up rather than scale up). US 2014/197034 does describe a desalination system that operates however on a different principle to the present invention. In addition, the plastic films of the conveyor belts in US 2014/197034 simply act as substrate for the flexible electrodes which are in direct contact with water. No watertight system is mentioned in this document.
U.S. Pat. No. 4,073,712 describes a system wherein, by means of an electrical field, water is treated and purified. This system is not used for desalination but rather more to remove colloidal particles and to prevent formation thereof, which amounts to an essential difference with the present invention. The electrodes creating the electrical field are coated with a plastic layer. However, their form differs from those used by us. In addition, the applied voltages in the order of several kV are much higher than in the present invention (less than 100 V) which amounts to another essential difference. In addition, U.S. Pat. No. 4,073,712 does not comprise a two-flow separation system, one of saline water and the other of desalinated water, leading to yet another essential difference with the present invention which comprises a novel separation device, a coaxial separator having an original bevel to obtain optimal continuous separation of the two cited flows. U.S. Pat. No. 4,073,712 concerns a fully different application from the present invention and its construction differs fully from that of the present invention.
The difference between the present invention and the prior art is that said first and second cathode electrodes 11A,11B and said first and second anode electrodes 12A,12B each respectively have a first surface 11F,11G,12F,12G in contact with air and a second surface 11E,11H,12E,12H opposite said first surface, said second surface being in direct contact with a wall in plastic 13B,13C,13A,13D, said plastic wall 13B,13C,13A,13D being in direct contact with the saline fluid 2 (FIG. 1).
The technical effect associated with this difference is that of reducing energy needs compared with the prior art (see Table 1).
On the contrary, DE 20315557 discloses an electrode fully coated with a ring in plastic, said plastic being in contact with the saline water, and US 2014/0197034 discloses a device with conveyor belts that is not impervious to sea water which means that at least one longitudinal surface of the electrodes is in direct contact with the sea water.
No document shows the combination of the following technical elements:
saline water-plastic wall-electrode-air.